mikey D

Hey guys, I posted over at mazspeed and it seems like no ones checking over there much these days.

I had a seriouse problem racing last weekend that has never happend before.
the only thing i changed on the car was wheel/tire combo since my last few races.
wider wheels and stickier bigger tires.

Cars an 85 GSL SE

1st Problem is now the rears lock up REALLY early. braking and turning at the same time require far more effort than ever before..

2nd problem and the one that really bothers me happened a few times on the same run. the rear of the car felt like it locked up and started shuddering.

it felt like i had massive wheel hop and the car became very difficult to control.
it happened on a hard 180 degree turn just after i let off the brakes and at the end of a slolam. I barely kept the car under control both times and it slowed me down tremendously.

Any ideas???

mikey D

i was also wondering if there was a way i could get the car to understeer a little.

the rear slides and the front stays planted.

yelwoci

Mikey
Often with brakes and tyres the problem is at the other end to the 'evidence'.
I have a 1982 Grp-1 12A racer here in UK, though not raced it for 5yrs.
If you have increased the braking power at the front, by virtue of more front end grip, you may cause more load transfer to the front, reducing the load at the back.
You cannot make significant changes in isolation..unless you are testing..for limits such as this.

Rear Shuddering is either caused by the effect of the road surface on locked/ing tyres or by vilent reaction withing the bushes, Typically on the RX the top link bushes, or a reaction in the prop/drive shaft (gbox-to-axle - we use different terms in UK)

The steering heaviness is inevitable with bigger tyres, but may be made worse by incorrect offsets.

My car has a split prop/drive shaft with a centre bearing at the same longitudinal possition as the foward chassis point on the lower links.

How about more info
What tyres and wheels/geometry/bushing/dampers/springs/ride height/brakes disc/drum

Regards
IanC

31rx7

What Ian said. He is right on.

You need to at a minimum reduce your rear braking via a rear bias adjuster.

You should also look at your spring rates, going to higher rates all around to compensate for added grip the wider wheels and tires are giving you. Of course, you will also likely need to go to better shocks.

It has been stated a 1000 times on this forum: get Susco's book on tuning the 1st gen suspension. www.gforceengineering.net.

yelwoci

"It has been stated a 1000 times on this forum: get Susco's book on tuning the 1st gen suspension. www.gforceengineering.net."

I've got it and can also highly recommend Jim's book..he also gives excellent feedback on the phone if your bought the book.
Whats he does at the rear end, which is key to much of his setup, isn't allowed in many racing rules (ie the ones I race with!). But its seriously worth it (USD150 I think)
IanC

wrankin

Question: when you say "racing" are you referring to track or auto-x? Also what size and brand of tires were you running before and after?

Brake bias can be taken care off with an adjustment valve. Alternately, you can try and run some stickier brake pads up front (or less sticky in the rear).

Under/over steer can be mildly adjusted by tire pressure and perhaps camber - are you running any negative camber on the fronts?

The rear noise is interesting - any indication of rubbing with your new tires and wheels? Was the shuddering happening during hard turning, mild turning or straights?

speedturn

iTrader: (1)

Hey Mikey, I used to be on the Mazspeed forum, but like you said it kind of died out this year, so I stopped checking over there daily.

Like yelwoci said, now that you have more traction with your better tires, the result is more weight transfer to the front, so now your rears are locking up. If you ran on a slick, rainy track without much grip, you would probably find that the opposite would occur: on a slick track, your fronts would lock up.

The solution to your rear brake locking problem is to either add more front brakes or less rear brakes.
The cheapest solution to have less rear brakes is to add an adjustable brake proportioning valve in the rear brake line. Mount the valve in the cockpit where you can reach it. It will require replumbing the rear brake line as it runs from the master cylinder to the rear axle, but this is probably the cheapest solution for a do-it-yourselfer like you. The adjustable proportioning valve acts kind of like a linear pressure reducer, which will reduce the hydraulic pressure going to the rear brakes. By keeping the **** close to the drivers seat, you can make a braking test run, then turn the **** a few times, then make another run, etc until you have the brakes balanced out. When testing lockup point, it really helps to have an outside observer watch you and tell you which wheel is locking up first. As long as you don't slide for long distances with locked up brakes, you will not flat spot your tires.

Other methods of making more front / less rear braking power:
different front calipers with bigger pistons
larger diameter front brake rotors
different rear calipers with smaller pistons
smaller diameter rear brake rotors
larger diameter rear tires
The problem with these methods is they are not very adjustable to fine tune brake balance
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The shuddering is probably from the rear brakes being on the verge of locking up then unlocking. I hear that on my race car when I am in a down hill braking zone (more weight has shifted to the front, giving the rear less traction, and the rear is on the verge of locking up and unlocking.)
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To make your car understeer,
add bigger front sway bar
stiffer front springs
softer rear springs
you should not use a rear swaybar on your 1st gen

mikey D

Hi speedturn, nice to see your still posting somewhere at least!
Ive really gotten hooked on autocross this year.
Im going to try and compete for points next year. My times are pretty competitive this year with a much less prepared car than many i am competing with.
I have been preparing my turbo car to go fast and be reliable.




Thanks everyone heres answers to your questions.

I do have Jims suspension book. Its awsome.

Im running a GSL SE so i have the rear disc brakes.
the car is gutted.

Autox is what im doing

15x7 wheels
225/50/15 tires made by GT Radial (inexpensive) 260 tread wear. They stick ALOT.
I never once broke em loose and ran faster than I had ever run before in this car.

spring rates- Super light, its a street car i dont plan on keeping past the next few events.

185 front
110 Rear

Koni adustables in front- set to full hard
High pressure gas type shock in rear.

Stock sway bars. Im going to take the rear bar off a the next event and see if i can generate more rear traction.

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to help with rear lockup
Im going to try some agressive front pads and run the stock rear pads.
Would that work ok? Any reccomendations on a pad that has really high braking power and lower autox temps?


to try and get some understeer
Im going to unhook one end of the rear bar at the next event and see if i can generate more rear traction.

TrentO

I ran a variant of Spec 7 for quite a few years on a very tight budget and within some limited rules. I used brake pads to control the bias, just as you mention. For orad racing I ran PBR Metal Masters in the rear and Hawk Blues up front. For autocross I'd go Metal Masters up front and cheap oem pads in the rear.

We also dumped the rear sway bar as part of the el-cheapo (yet within the rules) suspension tuning. This makes the car much more consistent. with the rear bar attached it always seemed the rear suspension was working all around, never finding a comfortable balance. With the bar removed, the initial roll of the rear was larger, but it settled into a steady state quickly and smoothly.

The biggest things you can do with a stock 1st gen to make it handle cheap are:
1) Put as much negative camber up front as you can. With slotting the mounts, this is about 1-1.5 degree.
2) Dump the rear sway bar.
3) Loosen up the watts link in the rear. Pull it apart, lube it up and double nut it on the diff housing. It should be loose enough to move easily, without any slack. if it thunks, it needs to be tighter.
4) Pull everthing that isn't welded down out of the car. Light cars acclerate faster, corner and brake better.

After this it starts to get expensive.

Another potential contributor to the rear hop is the increased weight of the new wheels / tires relative to your damper rate. This factor would be a fairly small contributor, so for all intents I would ignore it.

hope this helps,
-Trent

mikey D

the wheels weight diffferance from stock is marginal.

I described the condition as wheel hop but it happened at very strange times to be wheel hop. it nearly caused me to spin out 2 times. its so bad spectators noticed it when it happened. they said the whole rear appeared to shudder.


ive pretty much done your steps 1-4.
i lubed up all the rear suspension bushings.

Kentetsu

Are you sure that the new tires/wheels aren't rubbing somewhere? Either all the time, or just under hard cornering?

peejay

I have the opposite problem. When I try to accelerate in a corner the front tires shudder and slide across the road.

Very tight diff, high rear ride height, 3 link suspension, and 150lb front springs. Lots and lots of rear end bite, just right for dirt. On pavement, plow city.

mikey D

no rubbing.. i checked all 4 tires and fenders carefully to make sure.

To keep from rubbing im probably pretty close to stock ride hight.


i wondered if there was moisture in the rear calipers brake fluid, could it vaporizing cause them to lock up??

the car sticks like glue. did i mention that in any earlier posts?
I did not notice the tires spin or slide during the whole event except when i locked the rear brakes of course.

yelwoci

peejay
"When I try to accelerate in a corner the front tires shudder and slide across the road. Very tight diff..."

Thats your problem.. on tarmac/pavement. Rear end is sticking due to the diff. In my experience the RX's torque delivery is so consistant you can't unstick the rear with out having lots of power and/or being very agressive with steering and throttle inputs.
It is better to have a ligt load LSD. A typical 200hp 4-pot engined 2000lb car would have a plate diff set to about 60lbft. I run the RX SA-22 at 45-50.
This make the tail more controllable without being overly aggressive.
Haven't tried it on dirt though!

yelwoci

Mikey
Its all about balance.
You say it sticks like glue..but which end unsticks first?
Light front springs and heavy damping in generally is the wrong way to go.
If you've got simple 1 way adjustable units you're affecting rebound as much as bounce.
So you stand on brakes...load transfer to front...max braking at front GOOD..but at you release the brakes the the dampers hold the front down which causes other affects.

Replace you brake fluid after and before (if longer than 7 days) each race. Its a cheap maintenance job. Reduces corrosion and flushed water out. However it normally causes spongeyness and lack of braking.

The back end of an RX-7 (I, II, III) is a terrible design. If you overload it without upgrading parts you're asking for trouble. You could even be bending your damper rods, which wont do much for transient load changes.
Having said the tyres you

For what ever reason you chose the route, The optimum way to set a car up with respect to springs and dampers it to have the setup as soft as is practicable without losing feel of the car. Too many racers change their springs from say 100lb to 600lb and think they're going faster! The trick is to be soft enough to allow the tyres to accurately follow the road, while controlling the load transfer in transient load conditions.

So what have you got?
################################################## ###
15x7 wheels
225/50/15 tires made by GT Radial (inexpensive) 260 tread wear. They stick ALOT.
I never once broke em loose and ran faster than I had ever run before in this car.
################################################## ###
These wheels and tyres will be signifantly heavier and with stiffer effective tyre spring rates. The only need for bigger wheels is bigger brakes. On "standard" suspension low profile doesn't by you much. I race mine (250hp) on 195x60x13 (6.5JJ). Also allows you run cheap racing rubber using 13". .. Tyres are cheaper too - BFGR1s, Yoko's, etc.

spring rates- Super light, its a street car - Then live with the compromise ist not a race car <g>

185 front - Much to soft for the wheel/tyre combination. I run 325 wet 375-400 dry
110 Rear - Again this needs to be closer to 160-180 (but what ever you maintain some balance with the front - so you want the front to go up in stiffness more than the rear). don't remove the rear rollbar/stabilizer at these spring rates. 3deg roll and the rear end locks up and can bend bits.

Koni adustables in front- set to full hard - Almost never run dampers outside of the middle of their range. In this case back them off Koni D-sports can be damaged if you lock them up hard - They also need maintaining and servicing.
High pressure gas type shock in rear. - If they're working and not bent ... no problem on a standard rear end.

If you throwing the car away don't bother spending anything else on it, but some of the suggestions you've had above might work except one.......

A. Rear Brake Bias Valve - Great simple idea $100 simple to plumb in - non cockpit adjustable ones can be under the car, or in the engine bay. Cockpit versions are useful for adjusting in rain etrc. Watch plumbing design to avoid airlocks

B. Smaller Front Tyres or over pressure them. You should be running 225/50/15 at about 40psi hot for a road tyre (34 ish cold I guess) Push them up to 44psi hot. this is to compromise front grip...marginal idea

C. Brake Pads - DO NOT PUT PERFORMANCE PADS IN THE FRONT AND STANDRADS IN THE REAR!!! Unless you like spinning out lost. First corner of the race approaches...100mph...brake hard (the fronts start to warm up, the rears are at full braking force..cost they're at operating temperature, the fronts are now at 80% (way beyond standards would do, the rears are cooked and going off. if you're now off the brakes you lucky, else the tail has no brake stabilisation and you'll be untidy or worse. Next corner, fronts are working well, rears start to work but are starting to glaze, especillay coping the extra braking allowed by the fronts. Eventually you end up with total glazed pads and possible wharped rotors.

PS its also worth using 'racing' rotors or cryogenically treating perfomance items.

D: Stiffern the front Springs

Ouch...my soap box has just collapsed...I've got some splinters to remove.............

Y

mikey D

thanks for the information,

I was going to put some hawk blues up front.
sound like it would have made the condition that really bothers me the most (the rear locking and sliding) even worse.
I dont think the front of the car is jacking down front the shocks.
they are 1 turn off of full hard actually.




The car literally has 2 races left before i sell it.
next weekend is a 2 day event. maybe ill just put a bias adjuster in.
the guy buying it would appreciate it im sure.


I have an 85 im racing for points next year. the nature of the modifications ive done to it require the 15x7 wheels or I would be running on 13's.
the tires are just for EFI tuning purposes on next years car and will be replaced with a kumo victoracers or 710 next season.

i just tossed em on this current car so i could have something better than stock without spending any money.

peejay

I can torque my lug nuts with the tire off of the ground.

Before I readjusted my 3 link, I would spin the inside tire on tight turns. Not cornering, just turning. Now that I added some anti-squat, it drives the inside tire down in the turn (peculiarity of how the instant center is different inside vs. outside) and it doesn't even spin the tire anymore. Great for keeping a low profile, instead of laying rubber everywhere.

On dirt it's just plain awesome. At the last meet I was launching at 7k and the tires would spin for about 20 feet, engine speed not dropping one whit. Then 8k comes up in a blink and second gear. Brutal acceleration. I love it!

I couldn't trust a loose diff, the handling might be nicer off throttle but then it gets too unpredictable. Don't know if it will want to understeer or oversteer until it's too late. Not good if you're trying to eke out every last bit from the course!